1. Field of Invention
The present invention relates generally to sealing assemblies for rotary shafts located at least partially within a housing.
2. Description of Art
Industry is faced with the problem of transporting various media, such as abrasive-laden fluids/slurries, including, but not limited to, cement, paper pulp, oilfield drilling fluid, bitumen slurry, mica slurry, phosphate slurry, wastewater, crude oil, corn slurry and dredged sand and mud. Various types of equipment are used to handle the abrasive-laden fluids/slurries, such as but not limited to centrifugal pumps, progressing cavity pumps, paper pulp agitators, paper mill tower scrapers, and agitator gear boxes. Rotary seals are often a critical problem in such equipment.
Centrifugal Pump Basics
Centrifugal pumps are one of the most common types of equipment used for handling abrasive-laden fluids and slurries, and their construction is well-known and even somewhat standardized in several instances, and is not illustrated here (For a typical example, see Mission Fluid King Bulletin No. M203-3). They have been manufactured by various companies such as but not limited to Goulds, Gardner Denver, National Oilwell, Griswold, Twentieth Century and Duncan Machine. Centrifugal pumps utilize a shaft-driven impeller that rotates within a casing. Fluid enters the casing near the shaft centerline, and vanes on the rotating impeller rotate the fluid at high velocity, causing it to exit the casing at a discharge outlet. Sealing the rotating shaft where it penetrates the casing is a significant problem when abrasive-laden slurries are encountered.
Oilfield Cement Pump Seal Problems
In oilfield cementing operations, the shaft sealing arrangement on the centrifugal cement pump is typically a set of lip seals. The lip seals facing the cement are lubricated and flushed by a pressurized lubricant supply, but often fail prematurely due to the difficulty of regulating the pressure and lubricant flow rate across the lip seals. The lip seal facing the atmosphere fails quickly because it is ill-suited to retain the pressurized lubricant supply. If seal failure stops a cementing job, the cement contractor typically has to pay for drilling rig downtime until another contractor can show up and finish the job, and the rig rate is typically several thousands of dollars an hour. This means that operators usually try to continue operations, despite the ecological implications of leaked oil and cement associated with seal failure. Other expenses associated with seal failure include the cost of replacement seals, and the cost of the labor and equipment downtime associated with seal replacement.
Paper Pulp Stock Pump Seal Problems
Mechanical face seals are often used to seal centrifugal pumps that are used in paper mills to transport paper pulp, which is an abrasive slurry. The vast majority of pumps are the 3 to 4.5″ shaft size at 1200 to 1800 RPM, and typical pump flow rates are 300-1000 GPM.
Flush water is used to cool the mechanical face seals, and to dilute the abrasive concentration at the seal location; the flush water goes into the paper pulp mixture. A flush water flow rate of 4 to 5 gallons per minute is typically used, which means that a single pump requires 2.1 to 2.6 million gallons of water per year of operation. Since a typical paper mill uses hundreds of centrifugal pumps, the cost of purchasing the flush water, extracting the excess from the paper pulp, and cleaning it for disposal are enormous. When seal failure occurs, it is sometimes catastrophic, due to face breakage which can dump hundreds if not thousands of gallons of paper pulp stock onto the floor before the problem is identified. If seal loss results in shutting the production line down, the loss can be in the range of $50,000 an hour while the seals are being replaced.
The mechanical face seal is typically mounted to the stuffing box of the pump, but the stuffing box is often misaligned with the shaft, particularly on larger pumps, due to wear and corrosion of the piloting surfaces. Misalignment of up to 1/16″ has been observed in service.
The plants typically try to maintain a flush water pressure that is about 20 PSI higher than the stuffing box pressure under normal operating conditions (e.g. non-deadheading conditions) so that the flush water goes from the water supply into the pulp stock, rather than vice-a-versa.
Deadheading (e.g. closing the pump discharge line) occurs if too much stock is being pumped. Deadheading a pump can cause enough stuffing box pressure to cause reverse flow of the flush water, causing the pulp stock (e.g. process fluid) to enter the water supply system and contaminate or plug the water supply to other pumps, resulting in propagation of seal failures across the plant. The plants should, but don't always, use check valves to prevent the deadhead-related reverse flow. Deadheading also causes pump cavitation and shaft vibration that can shatter mechanical face seals, which opens up a large leakage path to the atmosphere.
Prior Art Problems Pairing Kalsi Seals with Lip Seals
The solid cross-section hydrodynamic rotary seals based on U.S. Pat. Nos. 4,610,319, 5,230,520, 6,120,036, 6,315,302 and 6,382,634, commonly assigned herewith, which are commonly known by the registered trademark “Kalsi Seals”, have not had commercial success in pumps because they generate too much heat at the high rotary speeds typical to pumps. They are utilized successfully in relatively low speed oilfield down-hole drilling equipment such as mud motor sealed bearing assemblies and advanced rotary steerable systems. One reason such seals are suitable for such down-hole equipment is because the seal leakage rate associated with the seal hydrodynamic pumping action is very low, and compatible with the relatively small lubricant reservoirs that can be accommodated in such equipment.
When used in oilfield down-hole drilling equipment, the above-noted hydrodynamic seals are sometimes used with outboard lip seals that are intended to help to protect the hydrodynamic seals. This type of arrangement can be very problematic, because the equipment operates in deep well-bores that are filled with drilling fluid, and the weight of the drilling fluid causes the ambient pressure surrounding the equipment to be very high. If a lip seal is simply positioned outboard of the hydrodynamic seal without some means to balance the pressure to the ambient environment, atmospheric pressure is trapped between the two seals at the time of assembly. Therefore when the assembly is exposed to the high ambient pressure down-hole, the lip seal simply collapses from the high differential pressure, and fails.
One way to gain a slight amount of down-hole utility from the lip seal is to defeat its sealing function in some manner, such as by cutting the lip, so that the lip cannot attain a true sealed relationship with the shaft. This lets the pressure between the seals equalize to the ambient pressure by allowing any unfilled space between the seals to fill with drilling fluid. If the region between the seals is partially filled with grease at the time of assembly, this arrangement provides a dilution zone between the seals so that the hydrodynamic seal is, at least temporarily, exposed to a reduced concentration of abrasives. Such a seal arrangement is typically called a barrier seal arrangement.
Another way to gain utility from the lip seal is to provide a mechanism that pressure balances the region between the two seals to the ambient environment pressure. One such pressure-balancing mechanism is shown on Kalsi Engineering Drawing 300-33. An O-ring that is in axial compression in a deep tapered radial groove, moves radially inward to compensate the pressure between the two rotary seals, and moves radially outward to burp-off the leakage of the inner hydrodynamic seal. Although on drawing 300-33 the pressure-balancing mechanism is shown with two hydrodynamic seals, it has also been used to pressure-balance the region between a hydrodynamic seal and a lip seal.
Prior Art Hydrodynamic Diverter Seals
The commonly assigned U.S. Pat. Nos. 6,109,618 and 6,494,462 of Dietle, which arise from the same original application and which are incorporated herein by reference for all purposes, teach the use of angled diverting features for causing copious lubricant flow within a dynamic sealing interface for the recited purpose of lubricating and flushing the dynamic sealing interface. In U.S. Pat. No. 6,109,618 these angled diverting features are called “restrictive diverters”, and in U.S. Pat. No. 6,494,462 they are called “pressure manipulation features”. Hydrodynamic diverter seals that incorporate such angled diverting features have been developed to act as miniature pumps that can generate substantial flow and pressure. The prior art does not suggest the use of such seals to pressurize other rotary seals, or to lubricate other rotary seals, or to provide a contaminant flushing action across the dynamic sealing interface of other rotary seals.
It is desirable to be able to overcome the shortcomings described above. More particularly, it is desirable to have a rotary shaft sealing assembly in which one or more lip seals are pressurized, lubricated and flushed. It is also desirable to have a rotary shaft sealing assembly in which a seal may be used to prevent reverse flow during deadheading conditions. It is also desirable to have a rotary shaft sealing assembly having a seal to contain pressurized lubricant while withstanding a high rotary speed. It is also desirable to have a rotary shaft sealing assembly in which a seal causes a stuffing box to align on the shaft during installation. It is also desirable to have a rotary shaft sealing assembly having a protective weir and rotating cover assembly to protect the exposed air side of the seal.